minor symfony#8058 Added the command to install babel-preset for babel-loader (bogdaniel, javiereguiluz)

This PR was merged into the 3.3 branch.

Discussion
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Added the command to install babel-preset for babel-loader

While testing this bundle i was getting hit by an erorr that i can't find the babel preset es2017 which i solved it by installing.
I'm new to this webpack thing and to work like this with javascript. Some people might be in my position and try for hours to solve this. :D

Commits
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b9edef8 Minor reword
ab41baf Update babel.rst
e74e556 Added the command to install babel-preset for babel-loader

Author: xabbuh

frontend/encore/babel.rst

@@ -15,8 +15,9 @@ Need to extend the Babel configuration further? The easiest way is via
 
     Encore
         // ...
-
-        // modify the default Babel configuration
+        
+        // first, install any presets you want to use (e.g. yarn add babel-preset-es2017)
+        // then, modify the default Babel configuration
         .configureBabel(function(babelConfig) {
             babelConfig.presets.push('es2017');
         })